Hannes Baumann

Abstract: To better understand the role of the German Bight (GB) as a nursery area for juvenile North Sea sprat Sprattus sprattus we sought to determine whether the area may receive only locally or also externally produced offspring. We sampled juveniles during 3 trawl surveys in the GB in August, September, and October 2004 and applied otolith microstructure analysis in order to reconstruct their distributions of the day-of-first-increment-formation (dif). These were contrasted with spatial and seasonal patterns of sprat egg abundance in the GB and its adjacent areas, observed during 6 monthly plankton surveys. It was found that the majority of juveniles originated mainly from April/May 2004, coinciding with high spawning activity west of the GB, whereas spawning and larval production inside the GB peaked notably later, in May/June. This indicated that a large proportion of juveniles was produced outside the GB and transported subsequently into it through passive and/or active migration. Shifts to later mean difs from one survey to the next and length distributions indicative of the simultaneous presence of multiple cohorts, supported the notion that the GB is a complex retention and nursery area for sprat offspring from different North Sea spawning grounds and times. Later born juveniles had significantly faster initial growth rates than earlier born conspecifics, which was likely temperature-mediated, given the strong correlation between back-calculated growth histories and sea surface temperature as a proxy for thermal histories of juveniles (r2 = 0.52).

Abstract: Evidence of fishery-induced evolution has been accumulating rapidly from various avenues of investigation. Here we review the knowledge gained from experimental approaches. The strength of experiments is in their ability to disentangle genetic from environmental differences. Common garden experiments have provided direct evidence of adaptive divergence in the wild and therefore the evolvability of various traits that influence production in numerous species. Most of these cases involve countergradient variation in physiological, life history, and behavioral traits. Selection experiments have provided examples of rapid life history evolution and, more importantly, that fishery-induced selection pressures cause simultaneous divergence of not one but a cluster of genetically and phenotypically correlated traits that include physiology, behavior, reproduction, and other life history characters. The drawbacks of experiments are uncertainties in the scale-up from small, simple environments to larger and more complex systems; the concern that taxons with short life cycles used for experimental research are atypical of those of harvested species; and the difficulty of adequately simulating selection due to fishing. Despite these limitations, experiments have contributed greatly to our understanding of fishery-induced evolution on both empirical and theoretical levels. Future advances will depend on integrating knowledge from experiments with those from modeling, field studies, and molecular genetic approaches. xD;

Abstract: For stock assessment purposes, Atlantic cod (Gadus morhua) from the coastal and offshore regions off northern Norway is usually allocated to Norwegian coastal cod (NCC) or Northeast Arctic cod (NEAC) by internal morphological features of their otoliths. As this classification is subject to individual interpretation by otolith age readers, this study investigated an alternative objective approach for the separation of the two cod groups, using otolith shape analysis. Several hundred otolith samples from coastal fjord areas along northern Norway and from the Barents Sea were analysed by univariate shape descriptors and elliptical Fourier analysis (EFA). When combining uni- and multivariate descriptors and applying the otolith reader typing as reference, the classification score was 89% for NCC and 90% for NEAC. These results indicate that the internal morphology of the otoliths, evaluated by the age readers, is translated to a great extent to their outer morphology and that otoliths can be allocated to NCC and NEAC by their shapes with high certainty. When genetic typing data (Pan I marker) were used as reference, the classification scores were reduced to 83% for NCC and 76% for NEAC when combining uni- and multivariate descriptors and excluding heterozygotes. This implies that differences in otolith morphology cannot directly be linked to genetic structure. Differences in environmental conditions, however, seem to have a considerable influence on how otolith growth increments and consequently otolith shapes are formed. As the various fjord systems in Norway provide local habitats and as differences within the NCC with regard to genetic structure and life-history parameters had been found in earlier studies, variation of NCC otolith shapes between three coastal regions was also examined. The region classification scores for reader-typed NCC varied between 60% and 81%.

Abstract: To better understand recruitment variability in small pelagic fish like sprat, it is important to know when during the extended spawning season the successful recruits are predominantly produced and which environmental factors determine potential survival windows. Here, we inferred the temporal origin of 2-year classes (2002-2003) of western and central Baltic sprat by means of otolith microstructure analysis, and found that in both years recruits mainly originated from the summer months June and July. In both years, this period coincided with temperature conditions in the surface layer of >12 degrees C and peak seasonal abundance of the largest copepod stages of Acartia spp., the major prey item of sprat larvae. The peaks in seasonal sprat egg abundance, however, occurred in April 2002 and March 2003 and therefore about 1-2 months earlier than the long-term mean spawning peak of sprat in this area (end of May/beginning of June). We hypothesize that increased temperatures in the bottom layer of the Baltic, where the pre-spawning sprat stock concentrates during winter months, potentially caused this shift in sprat spawning patterns, although early spring temperatures and feeding conditions in upper water layers were still unfavourable for larval survival. Sprat recruitment, however, was comparatively strong in both 2002 and 2003, suggesting that summer born individuals had high enough survival rates to compensate for the spawning shift, possibly due to high summer temperatures, limited dispersion, and low predation mortalities by Baltic cod as the major predator of sprat. Recruits were on average younger in 2003 than 2002, yet length distributions in October were almost identical, likely because a period of substantially higher temperatures in July/August 2003 promoted faster initial (larval) growth of survivors. Given the strength of the 2003 year class, in spite of lower overall prey concentrations in 2003 than 2002 in the study area, our findings appear to emphasise the paramount importance of summer temperatures as the recruitment determinant in Baltic sprat.

Abstract: Growth patterns inferred from otolith microstructure analysis were compared between sprat Sprattus sprattus early juveniles (26-42 mm total length, L-T) collected in August 2003 in shallow coastal waters of the Kiel Fjord, and sprat recruits (60-95 turn L-T) sampled in October during a pelagic trawl survey of the western Baltic Sea. At the end of August, a sudden and very rapid decline in otolith growth was observed in early juveniles but not in sprat recruits. Laboratory results indicated that the early juvenile fish were starving prior to capture. Specifically, when transferred to the laboratory, otolith growth rates immediately increased in fish provided ad libitum food rations, while otolith growth of starved fish continued to decline in the same manner observed prior to field collection. In addition, the vast majority of juvenile sprat had empty stomachs on the sampling day. Given that juveniles and recruits probably experienced similar temperature conditions, the rapid decline in juvenile growth rates presumably resulted from very poor feeding conditions in nearshore waters. Starvation during the early juvenile period has not been documented before, but may, at least in the case of Baltic sprat, comprise a density-dependent mechanism operating in coastal nursery areas in some years.

Abstract: Otolith microstructure analysis and hydrodynamic modelling were combined to study growth patterns in young-of-the-year (YoY) sprat, Sprattus sprattus, which were sampled in October 2002 in the central Baltic Sea. The observed 'window of survival', approximated by the distribution of back-calculated days of first feeding (DFF), was narrow compared to the extended spawning season of sprat in the Baltic Sea (mean +/- SD = 22 June +/- 14.1 days) and indicated that only individuals born in summer survived until October 2002. Within the group of survivors, individuals born later in the season exhibited faster larval, but more rapidly decreasing juvenile growth rates than earlier born conspecifics. Back-calculated larval growth rates of survivors (0.48-0.69 mm day(-1)) were notably higher than those previously reported for average larval sprat populations, suggesting that the YoY population was predominantly comprised of individuals which grew faster during the larval stage. Daily mean temperatures, experienced across the entire YoY population, were derived from Lagrangian particle simulations and correlated with (1) detrended otolith growth and (2) back-calculated, daily somatic growth rates of survivors. The results showed that abrupt changes in ambient temperature can be detected in the seasonal pattern of otolith growth, and that higher temperatures led to significantly faster growth throughout the entire age range of YoY sprat.

Abstract: Recruitment patterns of Baltic Sea sprat (Sprattus sprattus) were correlated to time series of (i) month- and depth-specific temperature conditions and (ii) larval drift patterns inferred from long-term Lagrangian particle simulations. From the latter, we derived an index that likely reflected the variable degree of annual larval transport from the central, deep spawning basins to the shallow coastal areas of the Baltic Sea. The drift index was significantly (P < 0.001) correlated to sprat recruitment success and explained, together with sprat spawning stock biomass, 82% of the overall variability between 1979 and 2003. Years of strong larval displacement towards southern and eastern Baltic coasts corresponded to relative recruitment failure, while years of retention within the deep basins were associated with relative recruitment success. The strongest correlation between temperature and recruitment occurred during August in surface waters, explaining 73% of the overall variability. Together, the two approaches advocate that new year classes of Baltic sprat are predominantly composed of individuals born late in the season and are determined in strength mainly by processes acting during the late larval and early juvenile stages. However, prior to be included in recruitment predictions, the biological mechanisms underlying these strong correlations may need to be better resolved.

Abstract: Eggs and larvae of Baltic sprat Sprattus sprattus L. were collected during 14 cruises covering the spawning season in 2002 in Bornholm Basin. Main egg and larval production was in April, with a second small peak in June 2002. The in situ larval abundance was corrected for transport processes by hydrodynamic model runs. Corrected larval abundance estimates were compared to initial larval production to derive an index of larval mortality. This index suggested a much higher survival of summer-over spring-born sprat larvae, with pronounced differences in survival for larvae > 11 mm. Independent evidence for this survival pattern was gained by measured RNA:DNA ratios in sprat larvae hatched from April to July 2002 and was linked to temporal variability in potential prey abundance. We found higher mean but less variable RNA:DNA ratios in spring- than in summer-born larvae, indicating a strong selection for fast growth in April and May but a less selective environment in June and July. Zooplankton data revealed high naupliar concentrations of Acartia spp. (a key dietary component of sprat) in April and May, but very low concentrations of larger prey items such as copepodites or adults. In contrast, abundance of larger prey increased considerably in June and July. The results suggest that larger sprat (> 11 mm) in April and May 2002 may have been food limited and, therefore, had lower rates of survival, supporting the underlying hypothesis of size-specific, temporally limited 'windows of survival' linked to the availability of suitable prey.

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Abstract: We studied the effects of food level changes on otolith and somatic growth in postlarval Baltic sprat reared initially for a period of 11 days under zero, low, and ad libitum feeding conditions. During a subsequent 11 day period, feeding regimes were reversed in half of the low and ad libitum feeding treatments, and starved fish were re-fed ad libitum rations. Somatic growth rates under low and ad libitum food rations ranged between 0.15-0.22 mm day(-1) and 0.48-0.63 mm day(-1), respectively, and led to significant differences in length and weight between feeding regimes. Previously starved fish, however, grew only 0.25-0.28 mm day(-1) under ad libitum conditions. During the first period, significant linear relationships were found for otolith v. length and v. weight growth across all treatments. After changing feeding regimes, increment widths failed to significantly predict somatic growth for 9 days, after which a significant relationship between otolith and somatic growth became re-established. Recent otolith growth was a good predictor of fish condition after the first, but not after the second period. The results suggest that perturbations in environmental conditions can temporarily decouple otolith from somatic growth in postlarval sprat, which needs to be considered in field studies.

Abstract: We used otolith microstructure analysis to reconstruct the growth histories of larval radiated shanny (Ulvaria subbifurcata) collected over a 2-week period in Trinity Bay, Newfoundland. A dynamic 3-dimensional, eddy-resolving circulation model of the region provided larval drift patterns, which were combined with measurements of temperature and zooplankton abundance to assess the environmental history of the larvae. The abundance of juvenile and adult capelin (Mallotus villosus), the dominant planktivorous fish in this area, was monitored using five hydroacoustic surveys. The goal was to determine whether environmental histories are helpful in explaining spatial and temporal differences in larval shanny growth, measured as cumulative distribution functions (CDF) of growth rates. We found evidence for a selective loss of slower growing individuals and recognized considerable spatial differences in the CDF of larval growth rates. Consistent patterns in capelin abundance suggested that faster growing survivors, sampled at the end of the 2-week period, developed in areas of low predator densities. A dome-shaped relationship between temperature and larval growth was observed, explaining a significant but small amount of the overall variability (14%). Effects of experienced prey concentrations on larval growth rates could not be demonstrated.